Arcane is a faction. Its bonus is Arcane Surge: Arcane units deal more skill damage.
Who gets it- Arcane units only
Bonus by tier
Distinct member classes on the field
2 classes- Skill damage +12%
3 classes- Skill damage +20%
4 classes- Skill damage +30%
Buy the Arcane upgrade for 40 Originite. It needs the Crest of Alliance upgrade and Deep Cave cleared on Normal. 
Flame FighterRare Warrior, 2 stars
FlareRare Warrior, 3 stars
FlamecallerRare Mage, 2 stars
SalamanderRare Mage, 3 stars
DivinerPrism Mage, 2 stars
OraclePrism Mage, 3 stars
PhasePrism Cavalry, 2 stars
Chrono WalkerPrism Cavalry, 3 stars
FrostbladeRare Duelist, 2 stars
Frost EmpressRare Duelist, 3 stars
